The first face milling cut of raw castings is a challenge because the outer shell of the casting is hard and frequently carries embedded moulding sand which abrades the cutting edges and rake faces of the milling cutters. The problem is further accentuated in the milling of cast iron engine blocks, where efforts at weight reduction and engine-cooling efficiency have reduced wall-thicknesses to a minimum. To mill gasket surfaces on such complex parts and thin sections without significant break out where the cutting tool exits the cut typically requires sharper cutting edges, i.e., a relatively high degree of positive rake, which, in turn, weakens the cutting edge and renders it more vulnerable to the accelerated and selective dulling action encountered by that portion of the cutting edge in constant edgewise encounter with the harder and abrasive skin of the casting while in the cut. Chipping of the cutting edge at that depth is not uncommon.
The resulting loss of the preferred cutting geometry shortens the useful life of the tool, and results in substantially increased cutting loads. These in turn increase the workpiece deflection, reducing the accuracy of the machining operation. If not adequately monitored, the condition will cause excessive breakout at the exiting edge of the cut, and require scrapping of the workpiece.
This real problem is addressed by this invention by providing a progressively strengthened cutting edge along the depth of the cut, i.e., by the provision of a cutting edge land which increases progressively in width from the bottom corner of the positive rake face upwardly along the cutting edge to the full height of cut for which the insert is designed. By that measure, the cutting edge is strengthened where it encounters the hardened and abrasive skin of the raw casting while preserving the downwardly increasingly sharper edge for the heavier metal removal load beneath the outer skin of the casting to the full depth of cut.
The hardened skin condition which characterizes previously uncut cast iron occurs in other materials as well. Forged steel, for example, and hot-rolled slab typically incorporate a harder skin layer from the working of the material, while certain metals exhibit surface hardening merely from previous machining cuts. For such applications as well, although less often encountered, perhaps, than the machining of raw castings, the wider cutting edge land at the top of the milling cut is preferred to offset the greater wear experienced at that location.